Method for growing single crystals



March 18, 1969 TOSHIO IYNOGUCHI ETAL 3,433,602 METHOD FOR GROWING SINGLE CRYSTALS Filed April 22. 1966 FIG. 1

KC! (moi SrCl (mol /o) FIG. 3

TOSHIO INOGUCHI ICHI H IKO NIWA KATSURO NAKAZAWA BY t.

ATTORNEYS United States Patent METHOD FOR GROWING SINGLE CRYSTALS Toshio Inoguchi, Sakai-ski, Ichihiko Niwa, Osaka-ski, and

Katsuro Nakazawa, Sakai-shi, Japan, assignors to Hayakawa Denki Kogyo Kabushiki Kaisha, Osaka, Japan Filed Apr. 22, 1966, Ser. No. 544,605 Claims priority, application Japan, Jan. 29, 1966, 41/5,079; Feb. 16, 1966, il/9,190

US. Cl. 23-300 Int. Cl. 'C01f 1/00; B01j 17/00 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method for growing single crystals. More particularly, it relates to a method for growing substantially unstrained single crystals of inorganic metal salts.

For growing a single crystal from a melt of the substance to be crystallized, there have been known various methods (e.g. Obreimov method, Schubnikov method, Bridgman method, Stockbarger method, Czochralski method, Kyropoulos method, zone melting method, Verneuil method, flame fusion method). However, none of them can be favorably applied for the production of substantially unstrained single crystals of inorganic metal salts having a transition point in crystallographic structure. For instance, cuprous chloride has a transition point around 407 C. and a melting point around 422 C. Its crystal is formed in the zincblende structure at a temperature lower than about 407 C. and in the wurtzite structure at a temperature from about 407 C. to about 422 C. When the melt of cuprous chloride is cooled according to a conventional method as above, the single crystal in the wurtzite structure is first grown and, on passing the said transition point, the transformation to the zincblende structure takes place. Such transformation results in the occurrence of strain which may lead to the production of polycrystalline bodies. In fact, there has never been provided any substantially unstrained single crystal of cuprous chloride in such a size as can be practically used, notwithstanding that the valuable utility of the single crystal of cuprous chloride as a light beam deflector has been developed [Soref et al.: Electronics, Nov. 29, 1965, p. 56].

It has now been discovered that, from a melt of an inorganic metal salt having a transition point which is admixed with a melting point depressing agent in such an amount that the melting point of the inorganic metal salt is lowered below the transition point, there is grown a substantially unstrained single crystal of the inorganic metal salt. It should be noted that the above discovery is clearly differentiated from a technical conception present in the known method for growing a single crystal from a melt of the substance to be crystallized with the aid of a flux. In the known method, the flux is used in such a large amount that it plays a role just like a solvent in solution. In the present invention, the amount of the melting point depressing agent is small and normally less than about 10 molar percent in the mixture with the inorganic metal 3,433,602 Patented Mar. 18, 1969 salt. Namely, the melting point depressing agent in the melt of the inorganic metal salt is just like an impurity. It has also been discovered that, on crystallization, the use of a glass tube of which the inner wall is covered by a carbon film can prevent favorably the inner wall from wetting with a melt of an inorganic metal salt, such wetting occasionally causing the occurrence of strain and the production of polycrystalline bodies. It has further been discovered that, on crystallization, the use of a glass tube having a specifically formed bottom part for the selection of a crystal nucleus makes it possible to grow a single crystal while blocking the production of polycrystalline bodies. The present invention is based on these discoveries.

It is a basic object of the present invention to embody a method for growing a single crystal of an inorganic metal salt. Another object of this invention is to embody a glass tube for crystallizing a single crystal of an inorganic metal salt therein. A further object of the invention is to embody a substantially unstrained single crystal of cuprous chloride. A further object of the invention is to embody a process for preparing .a substantially unstrained single crystal of cuprous chloride. These and other objects will be apparent to those conversant with the art to which the present invention pertains from the subsequent description.

The present invention will be hereinafter illustrated in detail by taking as an example the production of the single crystal of cuprous chloride.

In general, the single crystal of cuprous chloride has heretofore been prepared by cooling gradually the melt of substantially pure cuprous chloride according to the Bridgman method or the zone melting method. The resultant solid is colorless and transparent and appears to be a single crystal. In truth, however, it is a collective block of numerous fine crystals. In addition, it has a large strain.

According to the present invention, the melt of substantially pure cuprous chloride admixed with a melting point depressing agent in such an amount that the melting point of cuprous chloride is lowered below the transition point is cooled so that the single crystal of cuprous chloride is grown. The thus grown single crystal of cuprous chloride is substantially unstrained, because it is crystallized out directly in the zincblende structure without transforming thereto from the wurtzite structure.

The starting cuprous chloride is required to be substantially pure. In other words, it is desired to be purified to such an extent that no turbidity is observed on melting. The purification may be effected by conventional procedures such as washing with various solvents (e.g. glacial acetic acid, acetone, ether), recrystallization from conc. hydrochloric acid, sublimation under reduced pressure and zone melting under reduced pressure.

The admixing with the melting point depressing agent may be carried out before or after melting the starting cuprous chloride.

As the melting point depressing agent, there should be used a substance having at least the following properties: (a) lowering the melting point of cuprous chloride; (b) causing no reaction with cuprous chloride; and (c) being readily segregated from cuprous chloride. For instance, a metal chloride being larger than cuprous chloride in lattice constant and radius of cation can be favorably employed. Specific examples of the metal chloride are sodium chloride, potassium chloride, calcium chloride, barium chloride, strontium chloride, cadmium chloride, lead chloride, rubidium chloride, silver chloride, etc.

The amount of the melting point depressing agent to be admixed should be at least the one which can lower the melting point of cuprous chloride below the transition point. However, the use of a large excess amount must be avoided, because the melting point depressing agent is difficultly segregated and contaminates the resulting crystal. A suitable amount may vary with the kind of the melting point depressing agent. When the said metal chloride is used, the amount may be ordinarily less than about molar percent in the mixture with the starting cuprous chloride. A practical and preferable amount is generally from about 2 molar percent to about 6 molar percent. For instance, the relation between the amount of potassium chloride admixed with the melt of cuprous chloride and the melting point of the mixture is as shown in FIGURE 1, from which it is clear that the addition of potassium chloride in about 2.5 molar percent to about 4.0 molar percent is preferred. Further, for instance, the relation between the amount of strontium chloride admixed with the melt of cuprous chloride and the melting point of the mixture is as shown in FIGURE 2, from which it is clear that the addition of strontium chloride in about 2.2 molar percent to about 3.6 molar percent is favorable.

The crystallization may be performed in such a tube usually employed for crystallizing a single crystal as made of quartz glass or hard glass.

On the crystallization, it is advantageous to use the glass tube that a carbon film is formed on the inner wall for prevention of wetting with the melt of cuprous chloride, the wetting occassionally resulting in the occurrence of strain on cooling which may sometimes lead to the production of cracks and polycrystalline bodies. The formation of the carbon film may be preferably efiected by decomposing an organic compound such as methane, ethane, ether, benzene or acetone on the inner wall of the glass tube while heating. Although the carbon film can be also formed by any other conventional procedure (e.g. application of colloidal black lead), such film is apt to be eliminated. It is also advantageous to use the glass tube having a specifically formed bottom part for selecting a crystal nucleus so that the production of polycrystalline bodies can be inhibited. Such glass tube consists of an upper part for growing a crystal and a lower part (i.e. a bottom part) for selecting a crystal nucleus, these parts satisfying the following requirements: (a) the former and the latter being formed as a body intervening a neck of which the opening has a diameter suitable for introduction of a melt and selection of a crystal nucleus; (b) the latter having at least one curve so that the former and the latter are not coaxial; and (c) the said opening being not within the solid angle viewed from the end of the latter along the wall of the glass tube. The cross-sectional views of two typical examples of the glass tube provided with the above requirements are shown in FIG- URES 3 and 4 wherein 1 is the part for growing a crystal, 2 is the part for selecting a crystal nucleus, 3 is the neck and 4 is the end of 2. By the use of such glass tube, only one crystal nucleus is selected from numerous crystal nuclei occurred at 4 through 3 and grown in 1 to form surely a single crystal. Thus, the use of the glass tube having a carbon film on the inner wall and a specifically formed bottom part as illustrated above is the most preferred.

The cooling manner per se may be effected according to a known method (e.g. Bridgman method, Stockbarger method).

By application of the present invention, there can be readily obtained the substantially unstrained single crystal of cuprous chloride in such a size as practically utilizable (e.g. cylindrical single crystal of mm. in diameter and 50 mm. in length). Prior to the present invention, it was reasonably presumed that the admixing with a melting point depressing agent might result in the contamination of the resultant crystal with the melting point depressing agent whereby the transparency is lowered and the electric conductivity is elevated so that a variety of advantageous characteristics f h crystal of cuprous chloride are lost. Contrary to the presumption, the single crystal of cuprous chloride actually prepared by the present invention maintains excellent transparency and low electric conductivity. The melting point depressing agent is readily segregated from cuprous chloride and, in the ultimate, solidified collectively at the end of the single crystal prepared.

The present invention has been hereinabove illustrated on the production of the single crystal of cuprous chloride. However, it is clear that this invention can be generally applied for the production of single crystals of inorganic metal salts in the entirely same manner as in the production of the single crystal of cuprous chloride.

Practical embodiments of the present invention are shown in the following examples.

EXAMPLE 1 (A) Purification of cuprous chloride Commercially available cuprous chloride (reagent grade) is washed with glacial acetic acid, ethanol and ether in order in nitrogen atmosphere and dried at 75 to C. in nitrogen stream. The resultant cuprous chloride is charged in a transparent quartz glass tube. After heating at 300 C. under reduced pressure for 5 to 8 hours, the quartz glass tube is sealed and subjected to zone melting purification with a zone temperature of 550 to 600 C. and a rate of movement of 8 cm. per hour. The color of the melt is dark green to green until passing about 10 zones and then becomes blackish brown to yellowish brown While passing further zones. Finally, the solid part is made colorless and transparent. The purification is accomplished by passing about 20 zones.

(B) Formation of carbon film in crystallizing tube An end of a transparent quartz glass pipe is sealed to form a bottom part as shown in FIGURE 3, the ultimate end shaping a cone of about 60 in vertical angle. The resultant tube is heated at 600 to 800 C. under reduced pressure and vaporized acetone is introduced therein. The acetone is decomposed to form a carbon film on the inner wall of the tube.

(C) Growth of single crystal of cuprous chloride The quartz glass tube prepared as in (B) is heated under reduced pressure to eliminate the air therein, charged with a mixture of cuprous chloride purified as in (A) and potassium chloride (the amount being 2.5 to 4 molar percent in the mixture), heated at about 300 C. under reduced pressure for about 5 hours and then sealed. In a vertical two-zone furnace consisting of an upper part of higher temperature and a lower part of lower temperature (usually employed in the Bridgman method), there is suspended the above quartz glass tube in such a manner that the cone-shaped bottom of the said tube is positioned at the upper end of the said furnace. The tube descends with a speed of 0.5 to 2.8 mm. per hour in the furnace of which the temperature in the upper zone and in the lower zone are respectively about 450 C. and about 350 C. and the temperature gradient between these two zones is adjusted to 30 to 100 C. per cm., preferably 60 to 70 C. per cm. When the ultimate end of the cone-shaped bottom of the tube reaches to a temperature corresponding to the melting point (about 390 to about 400 C.) of the content, the crystallization of cuprous chloride is started. The single crystal of cuprous chloride is grown with the descending of the tube. Almost all of the potassium chloride admixed is separated out at the final stage of the growth of the single crystal of cuprous chloride and solidified.

(D) Appearance of single crystal of cuprous chloride The thus prepared single crystal of cuprous chloride is colorless, transparent and substantially unstrained. The size of the single crystal is about 20 mm. in diameter and about 50 mm. or more in length.

EXAMPLE 2 The quartz glass tube prepared as in Example 1(B) is heated under reduced pressure to eliminate the air therein, charged with a mixture of cuprous chloride purified as in Example 1(A) and strontium chloride (the amount being 2.2 to 3.6 molar percent in the mixture), heated at about 300 C. under reduced pressure for about 5 hours and then sealed. In a vertical two-zone furnace consisting of an upper part of higher temperature and a lower part of lower temperature (usually employed in the Bridgman method), there is suspended the above quartz glass tube in such a manner that the cone-shaped bottom of the said tube is positioned at the upper end of the said furnace. The tube descends with a speed of 0.5 to 1.5 mm. per hour in the furnace of which the temperature in the upper zone and in the lower zone are respectively about 450 C. and about 350 C. and the temperature gradient between these two zones is adjusted to 20 to 50 C. per cm. When the ultimate end of the cone-shaped bottom of the tube reaches to a temperature corresponding to the melting point (about 390 to about 400 C.) of the content, the crystallization of cuprous chloride is started. The single crystal of cuprous chloride is grown with the descending of the tube. Almost all of the strontium chloride admixed is separated out at the final stage of the growth of the single crystal of cuprous chloride and solidified.

What is claimed is:

1. A method for growing a single crystal of cuprous chloride which comprises cooling a melt of cuprous chloride admixed with a melting point depressant that is non-reactive with and readily segregated from cuprous chloride, in such an amount that the melting point of the cuprous chloride is lowered below its crystallographic transition point from wurtzite form to zincblende form, the eutectic point of cuprous chloride with said depressant being below the transition point,

2. A method claimed in claim 1, wherein the amount of the melting point depressing agent is from about 2 molar percent to about 6 moler percent in the mixture with cuprous chloride.

3. A method claimed in claim 1, wherein the melting point depressing agent is a metal chloride which is larger than cuprous chloride in lattice constant and radius of cation.

4. A method claimed in claim 1, wherein the melting point depressing agent is potassium chloride.

5. A method claimed in claim 1, wherein the melting point depressing agent is strontium chloride.

6. A method claimed in claim 1, wherein the cooling is effected in a glass tube having the inner wall covered by a carbon film and comprising an upper part for growing a crystal of cuprous chloride and a lower part for selecting a crystal of nucleus of cuprous chloride, the said parts satisfying the following requirements: (a) the upper part and the lower part being formed as a body intervening a neck of which the opening has a diameter suitable for introduction of a melt and selection of a crystal nucleus; (b) the lower part having at least one curve so that the upper part and the lower part are not coaxial; and (c) the said opening being not within the solid angle viewed from the end of the lower part along the Wall of the glass tube.

7. A method for growing a substantially unstrained single crystal of cuprous chloride which comprises cooling a melt of cuprous chloride admixed with a melting point depressing agent selected from the group consisting of sodium chloride, potassium chloride, calcium chloride, barium chloride, strontium chloride, cadmium chloride, lead chloride, rubidium chloride and silver chloride in an amount of about 2 molar percent to about 6 molar percent in the mixture with cuprous chloride.

8. A method claimed in claim 7, wherein the cooling is effected in a glass tube having the inner wall covered by a carbon film,

References Cited UNITED STATES PATENTS 2,367,153 1/ 1945 Swinehart 23300 3,087,799 4/1963 Fahrig 2330O 1,836,427 12/1931 Allen 23-300 1,848,513 3/1932 Bunn 23300 1,984,763 12/1934 Rockstrott 23300 3,051,555 8/1962 Rummel 23-273 3,156,533 11/1964 Imben 23-301 FOREIGN PATENTS 775,817 5/1957 Great Britain.

NORMAN YUDKOFF, Primary Examiner. E. P. HINES, Assistant Examiner.

US. Cl. X.R. 2397, 305 

